This invention relates to television and, more particularly, to method and apparatus for enhancing the sharpness of reproduced television pictures.
The reproduction of geometrical details is an important and much-investigated problem in television. In the transmission and reproduction of television images a distortion known as aperture distortion arises due to the fact that the electron beam of a television camera produces a scanning spot of finite size. As a consequence of the response of a television camera to the average light intensity within an image area covered by a scanning spot at any instant, the sharpness of image borderlines of high contrast is reduced. Such aperture distortion has two components, a horizontal component caused by the finite spot size in the horizontal direction (the line direction), and a vertical component caused by the finite spot size in the vertical direction (the field direction). There have previously been devised useful aperture correction circuits, of various types, which compensate a derived television information signal for this "spread" of information. One such system, described in an article entitled "Improved Signal Processing Techniques For Color Television Broadcasting" by R. H. McMann, Jr. and A. A. Goldberg which appeared in the March 1968 issue of the Journal of the SMPTE, provides both horizontal and vertical aperture correction by generating both horizontal and vertical detail signals which are combined and added to the video signal to produce a video signal of enhanced sharpness content. To generate the horizontal detail signal, each element of picture information on a scan line is compared to the adjacent picture elements occurring before and after the element being acted upon. Since the video signal appears sequentially in time, it is necessary to store video information for periods equal to the duration of one and two picture elements, typically, about 150 and 300 nanoseconds. Using delay lines with appropriate delay, signals known as "left", "main" and "right" signals are formed, which when combined in judicious manner produce horizontal detail signals that are ultimately added to a delayed version of the video signal to produce a resultant video signal of enhanced horizontal resolution. Vertical detail signals are generated by an analogous technique wherein a single scan line of video is compared element-by-element with the scan lines immediately preceding and following it. In this case, it is necessary to store video information for periods equal to the duration of one and two horizontal scan lines; i.e., 63.55 microseconds and 127.10 microseconds, respectively, for NTSC television. Comparison of successive scan lines produce "top", "main" and "bottom" signals which are combined in predetermined proportions to produce vertical detail signals that are also ultimately added to delayed original video. The thus-produced horizontal and vertical detail signals are usually added or otherwise combined before being added to the original video and are generically referred to as "detail signals". These signals are added back onto the original signal in such a phase as to emphasize the difference between the lines compared, in the case of vertical enhancement, thus enhancing vertical transitions, and to emphasize the difference between the picture elements compared, in the case of horizontal correction, thus enhancing horizontal transitions. The amount of vertical and horizontal aperture compensation added to a particular television signal can be varied by controlling the overall magnitude of the detail signals added back onto the original video signal.
While the described system of image enhancement has, with only minor modification, been used up to the present with generally acceptable results, it has the drawback that the baseline clipper included in the enhancement path to avoid enhancement of noise, removes low amplitude detail signals as well. This is not all bad, however, because facial blemishes and background wall texture, are not enhanced while important picture lines that the program should convey, such as eyes, teeth and hair highlights, are enhanced. A more serious drawback of this prior system arises from the nature of the television picture; because it consists of interlaced fields the correction signals are not derived from adjacent, but, rather, from alternate picture lines. This results in pre-shoot and overshoot of the detail signals during transitions from black-to-white or from white-to-black which cause a visible transition zone on the screen giving the appearance that objects are surrounded with black and white margins of objectionable width, giving the picture an unnatural look. This "edging" is particularly objectionable since its width tends to be constant and, accordingly, more pronounced. if the image it surrounds is small. This is visually disturbing in situations where the camera is zoomed back from an object which has "edging" in that the object becomes smaller while the width of the "edging" stays approximately the same.